A conventional spindle means used in a machine center is shown in FIG. 1, which includes: a spindle A disposed in a spindle sleeve B having a plurality of helical or annular cooling grooves C sealed in a sleeve holder D1 of a Z-axis machine frame D, a feed port D2 formed in an outer portion of the sleeve holder D1 for supplying cooling medium or liquid into the grooves C surrounding the sleeve B for cooling down the heat caused by running friction of primary bearing A1 and secondary bearing A2, and a discharge port D3 formed in the other side of the sleeve holder D1 for discharging the heat-exchanged cooling medium.
A stream of cooling oil mist or gas may also be fed through the port D2, the grooves C and a gas-feeding hole B, radially formed in the sleeve B for cooling the bearing A1, A2 and then directed through gas discharge hole B2 of sleeve B and through the remaining coils C to be outwardly discharged from port D3 to prevent the thermal expansion caused by frictional heat of rotating bearings during cutting operation of which the thermal expansion may deviate the centering of a tool E or may influence the cutting precision, causing a cutting error of unexpected depth. At the end of spindle A, there is provided with pulley or gear set F for a speed-variating transmission driven by the motor G. A tool holding rod H is slidably held in the hollow spindle A for holding the tool E which is clamped by or released from a chuck E1 operatively controlled by an actuator J provided at an end portion of the spindle A.
In order to enhance the rigidity or strength of the sleeve B for supporting the rotational spindle A, the cooling grooves C can not be made so deeply recessed in the outside wall of the sleeve B and should also be further sealed by the holder D1 to thereby greatly reduce the cooling effect of the grooves C since they are unable to approach the bearings A1, A2 so near. Meanwhile, after performing heat-exchange in the grooves C the temperature of the cooling fluid may be raised to reduce its cooling effect when fed into the bearing environment. The frictional heat caused by the cutting operational load and the rolling operation of the bearings will not be eliminated or disspated to thereby cause a linear thermal expansion of the spindle A at an increasing length gradient of 1.3 micro meter per 100 mm length of a steel spindle per degree of temperature (Centigrade) raise.
For accelerating the processing time and for a fine processing surface of a work piece, the spindle may be rotated at a very high speed such as about ten-thousand revolutions per minute to easily increase temperature to cause linear thermal expansion which will effect a precision processing job.
The side-protruded transmission system to be driven by the motor G provided aside the spindle A may cause a sidewardly thrusting or pulling force to thereby possibly bend or twist the spindle A to influence the processing precision. The bearings effected by the single-side thrusting force may be deformed to also influence the precision of the cutting operation.
Another conventional spindle means is shown in FIG. 1A including a plurality of cutting tools T held on a carriage or disk to be engaged with the spindle A positioned above the tool disk. In order to prevent a contact or impact of the spindle head with the tool T, the spindle sleeve B should be formed as a cone shape tapered downwardly and the spindle head should be prolonged its length to thereby cause a longer linear thermal expansion by frictional heat caused during the running of the spindle means to greatly influence a processing precision.
The present inventor has found the drawbacks of the conventional spindle means of a machine center, and invented the present improved spindle means of a machine center having increased cooling effect and having a direct straight driving operation by a servo-controlled motor for preventing twisting of the spindle.